1. Field of the Invention
The invention relates to the field of noninvasive tissue constituent analysis. More particularly, the invention relates to a method and apparatus for minimizing spectral effects in NIR spectral measurements for noninvasive blood analyte determination attributable to tissue state variations.
2. Description of Related Art
Near infrared (NIR) tissue spectroscopy is a promising noninvasive technology that bases measurements on the irradiation of a tissue site with NIR energy in the 700-2500 nanometer wavelength range. The energy is focused onto an area of the skin and propagates according to the scattering and absorption properties of the skin tissue. Therefore, the reflected or transmitted energy that escapes and is detected provides information about the tissue volume that is encountered. Specifically, the attenuation of the light energy at each wavelength is a function of the structural properties and chemical composition of the tissue. Tissue layers, each containing a unique heterogeneous particulate distribution, affect light absorbance through scattering. Chemical components such as water, protein, fat and blood analytes absorb light proportionally to their concentration through unique absorption profiles or signatures. The measurement of tissue properties, characteristics or composition is based on detecting the magnitude of light attenuation resulting from its respective scattering and/or absorption properties.
While noninvasive prediction of blood analytes, such as blood glucose concentration, has been pursued through NIR spectroscopy, fluctuations in tissue state, such as skin temperature, lead to increased spectral variance that can lead to a reduction of the net analyte signal, thus rendering it difficult to extract valuable analyte information.
Human tissue can consist of as much as 80% water, which has a known peak shift that is a function of temperature, in the NIR absorbance spectrum. As temperature increases, the water band shifts to shorter wavelengths as a result of a decrease in hydrogen bonding. As light irradiates the tissue and travels through the layers of the skin, it is scattered and absorbed by the constituents of the skin before exiting the skin, where it is detected by a spectrometer. Skin temperature variation is introduced into the spectral measurement in two ways. First, the resulting signal contains spectral information from the tissue volume it has traversed, including contributions from the natural temperature gradients present in the optical sampling path of human tissue. Second, human skin and sub dermal tissue undergo temperature variations, as a result of environmental and physiological factors, to maintain a uniform core body temperature. During the course of a day, skin temperatures have been observed to fluctuate by as much as 5xc2x0 F. in healthy individuals. These factors result in temperature variations between the measurements comprising a data set. Therefore, water band shifts within a measurement and between measurements are present in the data set. The data set is used to estimate the analyte of interest through the development of a multivariate mathematical calibration model.
Within and between measurement temperature variations add a level of complexity to the multivariate analysis, making it more difficult to extract valuable analyte information. Large variations in temperature lead to increased spectral variance that can lead to a reduction of the net analyte signal. In addition, uncontrolled skin temperature variations have a higher probability of correlating with analytes of interest. Such chance correlations can lead to false calibrations, which may or may not be discernible.
Various spectroscopic methods and apparatuses that aim to monitor or alter sample temperature in some way are described in the prior art. For example, J. Braig, D. Goldberger, B. Sterling, Self-emission noninvasive infrared spectrophotometer with body temperature compensation, U.S. Pat. No. 5,615,672 (Apr. 1, 1997) describe a xe2x80x9cself-emissionxe2x80x9d glucose monitor that noninvasively measures glucose concentration in a subject""s blood by monitoring the infrared emission of glucose in the blood at long infrared wavelengths near 10 microns. The described device utilizes the infrared energy emitted by the person""s blood and/or surrounding tissue to perform the absorption analysis. A temperature-sensing device for measuring the person""s internal temperature at the arm is also used to adjust the constituent concentration measurement for temperature dependent effects. While the use of the person""s own infrared energy, emitted as body heat, for an infrared source eliminates the necessity of providing an energy source, the described device and the attendant method require a determination of the individual""s internal temperature; however, the sensor measures temperature at the skin surface. Therefore, the calculated compensation for internal body temperature to be applied to the measured spectral signal introduces a significant source of error in the analyte concentration estimate. Additionally, the sensor""s thermal time constant introduces an undesirable latency into the measurement, possibly as long as 1xc2xd minutes. Furthermore, the described device merely calculates a correction to be made to the spectral signal that compensates for the effect of the subject""s body temperature. No provision is made for control of temperature within a target range in order to provide an optimal sample temperature, at which temperature-related spectral effects are minimized. Additionally, the Braig, et al. teachings are concerned with the mid- and far regions of the IR spectrum.
M. Block, Non-invasive IR transmission measurement of analyte in the tympanic membrane, U.S. Pat. No. 6,002,953 (Dec. 14, 1999) describes non-invasive methods and apparatuses for measurement of concentrations of selected blood constituent in which an optical device is inserted into the external ear canal to direct a portion of the electromagnetic radiation onto an IR detection and analysis device. The tympanic membrane is cooled to create a temperature differential with the inner ear, thus facilitating the emission of thermal radiation across the tympanic membrane. The insertion of an optical instrument deep into the ear canal and chilling of the tympanic membrane are by no means invasive, although they may be seen to be minimally invasive compared to more traumatic methods of sampling, such as venipuncture. The Block teachings make no provision for spectroscopy-based measurement of temperature at the measurement site. The cooling of the tympanic membrane is not done to minimize spectral effects of tissue state fluctuation, but to facilitate thermal transfer. With the exception of cooling the tympanic membrane in a stereotypical fashion, the Block device is incapable of controlling temperature at the measurement site. The Block device further provides no closed loop in which spectroscopic temperature determinations provide the feedback required for control of site temperature.
J. Braig, C. Kramer, B. Sterling, D. Goldberger, P. Zheng, A. Shulenberger, R. Trebino, R. King, C. Barnes, Method for determining analyte concentration using periodic temperature modulation and phase detection, U.S. Pat. No. 6,161,028 (Dec. 12, 2000) describe a method of determining the analyte concentration of a test sample that employs a rationale similar to that of Block. A temperature gradient is introduced in the test sample and infrared radiation detectors measure radiation at selected analyte absorbance peak and reference wavelengths. The Braig, et al. teachings employ gradient spectroscopy, in which a temperature gradient is produced in the sample to facilitate thermal transfer, thereby delivering more thermal radiation to the radiation detectors. The method described does not address the problem of spectral effects related to fluctuations in tissue state at the measurement site and their confounding effect on the net analyte signal. With the exception of inducing a temperature gradient in the sample, the described method provides no way of controlling sample temperature within a target range that minimizes the spectral effects caused by fluctuations in sample temperature.
In view of the problems left unsolved by the prior art, there exists a need for a way to control spectral effects attributable to tissue state variations, such as skin temperature, during NIR-based, non-invasive blood analyte determination, by monitoring the selected tissue state parameter and maintaining it within a predetermined target range of values at which such spectral effects are minimized. It would be a significant technological advance to monitor the tissue state parameter spectroscopically, in which a calibration model calculates measured values by correlating shifts in the selected tissue state parameter with shifts in observed spectral effects, thus eliminating the time constants imposed by conventional sensor devices. It would be desirable to provide a means of controlling the selected tissue state parameter, driven by a closed loop, in which the calculated values provided the feedback for determining the degree of control to be applied.
The invention provides a method and apparatus for minimizing confounding effects in a noninvasive in-vivo spectral measurement caused by fluctuations in tissue state. A selected tissue state parameter is spectroscopically monitored through the application of a multivariate calibration model that correlates spectroscopic changes with fluctuations in the selected tissue state parameter, providing near-instantaneous measurements, without the imposition of any significant time constants. A target range of values for the selected parameter is empirically determined from an experimental data set by observing the spectra of the data set to determine a range of values at which spectral effects due attributable to the changes in the selected parameter are minimized. During measurement, the selected parameter is continuously monitored. The calculated values provide feedback in a closed loop that drives a device for maintaining the selected tissue state parameter within the target range. Means for both active and passive control of the tissue state parameter are included in the invention.
A preferred embodiment of the invention provides a method and apparatus for minimizing the confounding effects in a noninvasive near IR spectral measurement attributable to shifts in skin temperature at the tissue measurement site. Skin temperature at the measurement site is spectroscopically monitored by calculating temperature values through the application of a multivariate calibration model that correlates spectroscopic changes with shifts in skin temperature. Thermal time constants imposed by conventional temperature sensing devices are eliminated, providing near-instantaneous temperature readings. Active and passive control means are provided. Passive control is achieved through the selective application and removal of an occlusive thermal wrap. Active control is provided by a thermistor applied to the skin in the vicinity of the measurement site. Active and passive control may be applied in complementary fashion or they may be used separately. In a particularly preferred embodiment of the invention, the control means is incorporated into the measurement instrument, wherein the calculated skin temperature values provide the feedback in a closed loop that drives the control device. In an alternate embodiment of the invention, the temperature values are supplied to an operator, who then applies active and/or passive control to achieve and maintain a skin temperature within the target range. By monitoring skin temperature spectroscopically and employing methods of passive and/or active control it is possible to reduce the effects of skin temperature variation on the NIR measurement. The invention finds particular application in the noninvasive measurement of blood glucose concentration.